(1) Field of the Invention
The present invention relates to a hybrid fuel cell system applied to a fuel cell and a general cell, particularly a control device for an output current of the fuel cell, and its operation.
(2) Description of the Related Art
Fuel cells such as a phosphoric acid fuel cell, a molten carbonate fuel cell, and a solid oxide fuel cell, are constructed so that water and electricity are generated from the reaction of hydrogen and oxygen. Hydrogen supplied to such fuel cells is made at a reforming device from fuel such as natural gas, or methanol. Also a utilization factor of the fuel cell is usually controlled to be 80% or less, which contributes to gaining a smooth reaction of the fuel cell. The fuel cell constructed above is connected to an external load to supply electric power to the load. Increasing/decreasing hydrogen supplied to itself, the fuel cell can follow a fluctuation in the value of the external load to supply sufficiency electric power to the external load as long as the fluctuation is gradual. However, when the value of the external load changes suddenly, hydrogen cannot be supplied to the fuel cell as fast as it should be, causing the fuel cell to lack gas. This leads a decrease in an output voltage, making it impossible for the fuel cell to follow the change in the value of the external load to supply sufficient electric power to the external load. Also in such a case, quantity of exhaust gas generated at the fuel cell is so little that the reforming device cannot execute a reforming operation, causing a safety device to be driven to make an urgent stop of the fuel cell.
Responding to the above problem, a general hybrid fuel cell system has been adopted, wherein sufficient electric power is supplied to the external load even when the value of the load changes suddenly. Also this system adopting a storage cell for helping the fuel cell to supply electric power to the external load prevents an overuse of the fuel cell. The system basically consists of a series circuit wherein a DC/DC converter is inserted into an output current line, and the storage cell connected parallel with each other. Further, the fuel cell in the system is constructed to output around a certain amount of electric power at all times, except when the value of the load increases suddenly. When the value of the load increases suddenly, the storage cell equipped parallel to the fuel cell supplies electric power to make up for its shortage. Then, once the value of the load becomes stable, the storage cell stops supplying electric power. In other words, the amount of the output of the storage cell becomes 0, and electric power is exclusively supplied by the fuel cell.
The system is further explained referring to the example of an electric automobile adapting the system. That is, the hybrid fuel cell system applied to the electric automobile adopts the storage cell in addition to the fuel cell to make up for the shortage of electric power which occurs when the automobile accelerates or climbs a hill. On the other hand, only the fuel cell is used when the automobile travels a flat ground at a certain speed. When the system adopts both the storage cell and the fuel cell, the amount of electric power supplied by the fuel cell is controlled to be around a certain level by the DC/DC converter. Also the storage cell is charged with the same or smaller amount of electric power output from the fuel cell.
The general hybrid fuel cell system is, however, effective only when the value of the external load is larger than the output of the fuel cell. That is, for the external load with its value larger than the output of the fuel cell the system is valid since it adopts a follow-up electric power generation manipulating a storage cell to make up for the shortage of electric power. However, for the external load with its value smaller than the output of the fuel cell the use of the system provides the following problem. That is, using the DC/DC converter the output of the fuel cell is set to be around a certain level at all times in the system. Therefore, when the value of the external load is smaller than the output, some electric power (the difference between output of the fuel cell and the external load) needs to be consumed. Generally an internal load such as a heater is equipped inside the system to consume such electric power. This leads to a problem of lowering the energy efficiency since the amount of the electric power generated in the system is larger than the amount of the electric power necessary for the external load.